In the coming year, we will continue our studies on the effect of ultraviolet light on the epidermal melanocyte and cultured retinal pigment epithelial cells. We will devise a system by which psoralen together with UVL can be used in our organ culture and cell culture systems, in order to elucidate the mode of action and synthesis of tyrosinase, on the activation of tyrosinase within the melanosomes, and on the melanization of melanosomes. In the combined epidermal keratinocyte and melanocyte cultures, we will study the possible influence of psoralen and UVL on melanosome transfer and melanocyte-keratinocyte interactions. In this same culture system, we will study melanocyte-keratinocyte interactions at the electron microscopic level using freeze-fracture techniques. We will continue our in vivo studies on melanocyte-keratinocyte interactions in the guinea pig, with special reference to the effect of psoralen and UVL. We also plan to make use of a very interesting genetic variant in the mouse, the C57 dilute black, in which melanosome transfer seems to be impeded. We have found that in this system ultraviolet irradiation increases melanocyte number but does not increase, to any great degree, melanosome transfer. Since there are some indications that similar disorders in man may be associated with severe immunological deficiencies, a study of this system may be of great importance in the investigation of cell-to-cell interactions.